secure-logs : add implementation of FssAgg

Add implementation "Forward-secure sequential aggregate authentication.",
aka FssAgg, based on the work of Ma and Tsudik :
https://eprint.iacr.org/2007/052.pdf

Signed-off-by: Shahriyar Jalayeri <[email protected]>

pkg/newlog/cmd/fssagg.go

@@ -0,0 +1,82 @@
+// Copyright (c) 2024 Zededa, Inc.
+// SPDX-License-Identifier: Apache-2.0
+
+package main
+
+import (
+	"crypto/hmac"
+	"crypto/sha256"
+)
+
+// Notes and TODOs:
+// 1- save num key evolve in the batch metadata
+// 2- if a batch of log reaches the server it means compromise of content integrity, stream integrity and truncation is detectable.
+// 4- in the next version we can use the TPM emulated secure monotonic counter to truncation (deletion) detection.
+// 5- each log entry in a batch should have seq number so we can verify it.
+// 6- adding delete detection will in the next version will break logging?
+
+// This is based on wor of Ma and Tsudik:
+// Ma, Di, and Gene Tsudik. "Forward-secure sequential aggregate authentication."
+// IEEE Symposium on Security and Privacy (SP'07). IEEE, 2007.
+// https://eprint.iacr.org/2007/052.pdf
+
+// h must be a collision-resistant one-way hash function (here using we are using SHA-256)
+func h(data []byte) []byte {
+	h := sha256.New()
+	h.Write(data)
+	return h.Sum(nil)
+}
+
+// hsk must be a secure MAC function h (here we using HMAC with SHA-256 and a secret key)
+func hsk(key, message []byte) []byte {
+	mac := hmac.New(sha256.New, key)
+	mac.Write(message)
+	return mac.Sum(nil)
+}
+
+// fssAggUpd updates key using hash of previous key
+func fssAggUpd(prevKey []byte) []byte {
+	return h(prevKey)
+}
+
+// FssAggSig takes the current key, aggregated signature so far and a message,
+// calculates the message MAC, updated aggregated signature and updates the key
+// for forward security. It returns the aggregated signature plus the updated key.
+func fssAggSig(key []byte, aggMAC []byte, message []byte) ([]byte, []byte) {
+	var aggSig []byte
+
+	// generate MAC for current message and aggregate current MAC with previous MACs
+	curMAC := hsk(key, message)
+	aggSig = h(append(aggMAC, curMAC...))
+
+	// update key for forward security
+	nextKey := fssAggUpd(key)
+
+	return aggSig, nextKey
+}
+
+// FssAggVer verifies aggregated signature over messages, accepting the initial
+// secret key and the aggregate signature, returning a boolean indicating the
+// verification result.
+func fssAggVer(secretKey, aggSig []byte, messages [][]byte) bool {
+	key := secretKey
+	var computedAggSig []byte
+
+	for i, message := range messages {
+		// generate MAC for current message
+		curMAC := hsk(key, message)
+
+		// aggregate current MAC with previous MACs
+		if i == 0 {
+			computedAggSig = h(curMAC)
+		} else {
+			computedAggSig = h(append(computedAggSig, curMAC...))
+		}
+
+		// update key
+		key = fssAggUpd(key)
+	}
+
+	// this compare is constant time, so no time leak and attack.
+	return hmac.Equal(computedAggSig, aggSig)
+}